Patent GB-1160114-A (GB)
Title: Improvements in Electrical Signalling Systems
Inventor: CLARK ADRIAN PERCY
IPC: H04J1/00
Filed: 1965-10-08
Abstract: 1,160,114. Multiplex signalling. BRITISH TELECOMMUNICATIONS RESEARCH Ltd. 3 Oct., 1966 [8 Oct., 1965], No. 42821/65. Heading H4L. In a frequency division multiplex system a subcarrier is generated for each channel, each subcarrier is modulated with an information bearing signal using a first type of modulation, the modulation of each of a first group of modulated subcarriers being influenced by a respective one of a second group of modulated subcarriers, and a main carrier f c  is modulated with all the subcarriers using a second type of modulation. As described, the frequency spacing fm in cycles per second between adjacent subcarriers is arranged to be equal to the  signalling speed in bands per second as is also the spacing of each of the two subcarriers adjacent the carrier f c  from the frequency fc- The centre frequency f c  is arranged to have a voltage level n times greater than that of each data signal where n is the number of channels, the channels are transmitted in element synchronism and a timing signal is transmitted by phase-shifting the main carrier by 180 degrees at alternate signal elements. The value (1 or 0) of a signal element in an odd-numbered data channel (subcarrier of frequency f above the carrier frequency f c ) is determined by the phase-shift of that channel subcarrier relative to that of the main carrier f c  and the value of a signal element in an even numbered channel f 2  is determined by the phase shift of that channel relative to that of the channel f 1  which has the same frequency separation from the carrier f c . Thus there are four different phase conditions representing the four possible binary combinations for the two channels, two of the combinations being equivalent to an amplitude modulation of the carrier f c  and the remaining two to a frequency modulation of f c . The system is stated to minimize the effect of Doppler shift in, for example, satellite radio communication systems. Transmitter.-In one arrangement, Figs. 3 and 4, an oscillator 01 produces a square wave A at frequency f m  which is divided by four at stage DS to provide the synchronizing signal B which operates 180 degrees phase switches PS1, PS2. PS1 is supplied via a 90 degrees phase shifter with the carrier frequency f c  generated at 02 and PS2 is supplied direct from 02. The output from PS2 supplied via adding circuit AD3 is the central carrier containing the phase-modulated synchronizing signal. A sawtooth generator STG converts the signal f m  into a range of harmonically related frequencies fm, 2f m , &c. which are selected by filters BF1 &c. Considering channels 1 and 2, f m  is selected and passed via a level and phase control circuit LPC (curve D) to a 180 degrees phase switch PS3, and after a 90 degrees phase shift at PH1 (waveform E) to a 180 degrees phase switch PS4. Both waves D and E are phase shifted by 180 degrees when the signal on channel 1 is a 1 and are left unchanged when it is a 0. The outputs G and H from the phase switches are fed to a gate circuit G which is controlled by the signal on channel 2 so that when this signal is a 1 only signal H is passed and when it is 0 only signal G is passed. The resulting outputs K or L are fed via ADDING circuits AD1, AD2 respectively to product modulators PM1 or PM2, together with the signals from the corresponding odd and even channels. Modulators PM1, PM2 are suppressed carrier amplitude modulators producing upper and lower sidebands and the carrier input f c  to modulator PM1 is in phase with the transmitted carrier so that the phase modulated signals amplitude modulate the main carrier. In the case of modulator PM2 the phase of the applied carrier leads that of the transmitted carrier by 90 degrees so that the resultant formed by adding the output of PM2 to the carrier at AD3 is equivalent to the phase shift signals frequency modulating the main carrier. Thus when the signal on channel 2 is a 0 the combined channels 1 and 2 signal is transmitted as phase-modulation/frequency modulation and when it is a 1 it is transmitted as phase modulation/amplitude modulation. In an alternative transmitter, Fig. 2 (not shown) subcarriers lying either side of the central carrier f C  are provided by modulating f c  by f m  in a product modulator and are then phase-modulated by the appropriate channel signals. Receiver.-At the receiver, Figs. 5, 6, the received signal is supplied via a filter BPF and amplifier AGA to a detector DT for the timing signal C and also to an AM detector AMD providing the demodulated AM signal H, and to a FM detector FMD, Fig. 7 (not shown), providing the demodulated FM signal J. The timing signal C is converted to a sine wave at NBF and after phase adjustment at PC the resulting signal E is converted at FC into a sawtooth wave F which triggers a blocking oscillator BO to produce pulses G. The outputs of detectors AMD, FMD are added at AC to produce waveform K and applied to a coherent phase detector CPD receiving as a reference signal the waveform L of frequency 2f m  c/s. which is arranged to be of the same phase as the signal element 0 in channel 1. The reference signals for the remaining pairs of channels are <SP>3</SP>fm 4fm c/s, &c. the phase being correspondingly adjusted. After detection the channel 1 signal is integrated (waveform M) and supplied via a threshold circuit TLD (waveform N) to a bi-stable circuit SB1 which reconstitutes the original signal P of channel 1. The outputs of detectors AND, FMD are also fed to phase detectors and integrators CPD2, CPD3 respectively which invert and clamp the signals to remove negative-going pulses and provide outputs Q and R. Output Q represents amplitude modulation of the main carrier f c  and thus a signal element 0 in channel 2 and output R represents frequency modulation and a signal element 1. This is determined by circuit LC which controls bi-stable circuit SB2 to reconstitute the original signal T of channel 2.